Anatomy 12.4-12.7

Inactivation state

Although the activation gate is open, the inactivation gate is temporarily closed (for several milliseconds) following activation of the Na+ channel—during this time, it cannot be stimulated to reopen, and entry of Na+ is prevented. (The resting state of voltage-gated Na+ channels are reestablished as the inactivation gate opens and the activation closes.)

Resting state

Although the inactivation gate is open, the activation gate is closed, and entry of Na+ is prevented.

time between the neurotransmitter release from the presynaptic cell, its diffusion across the synaptic cleft, and neurotransmitter binding to receptors in the postsynaptic plasma membrane.

Describe Synaptic delay?

At rest, a neuron has a -70mv RMP. All gated channels are closed, and concentration gradients exist for Na+, K+, and Cl− along the length of the neuron, and for Ca2+ at the synaptic knob. The RMP is established and maintained by K+/Na+ leak channels and pumps

Describe a neuron at rest?

One oligodendrocyte can myelinate 1 mm of multiple axons each at multiple spots. No neurilemma formed and neurofibril nodes form between adjacent wrapped segments

Describe myelination is the CNS?

negative

Fill In The Blank: Cytosol is relatively ______ compared to IF

K+

Fill In The Blank: Higher concentration of ____ in cytosol verses interstitial fluid (IF)

Na+, Cl-, Ca2+

Fill In The Blank: Higher concentrations of _____ in IF than in cytosol

the actions of pumps

Ions are unevenly distributed across the plasma membrane due to what?

Myelination

process by which part of an axon is wrapped with myelin.

Glial cells

small mitotic cells found within both the CNS and the PNS. They don't transmit electrical signals, but they do assist neurons with their functions. They cooperate to physically protect and help nourish neurons as well as provide an organized, supporting scaffolding for all the nervous tissue. During development they guide young, migrating neurons to their final destinations.

Synapse

specific location where a neuron is functionally connected either to another neuron or an effector (muscle or gland).

Channels

Protein pores in the membrane that allow ions to move down their concentration gradients (into or out of the cell), when open they allow a specific type of ion to diffuse

both sodium-potassium (Na+/K+) pumps and calcium (Ca2+) pumps. A great deal of energy is required to power the vast number of Na+/K+ pumps in a neuron's plasma membrane—approximately two-thirds of a neuron's energy expenditure!

The plasma membrane of neurons contains what kind of pumps?

when they are stored in synaptic vesicles and released from the synaptic knob of a presynaptic neuron into the synaptic cleft. Some of the neurotransmitter diffuses across the synaptic cleft to bind to receptors within the plasma membrane of the postsynaptic neuron to initiate a graded potential.

Transmission between a presynaptic and postsynaptic neuron occurs when neurotransmitter molecules do what?

Neurolemmocyte wraps around a portion of an axon then cytoplasm and plasma membrane begin to form consecutive layers as wrapping continues. The overlapping inner layers form the myelin sheath, thus pushing the cytoplasm and nucleus to the outside of cell forming the neurilemma

What are the PNS myelination steps?

Leak, chemically gated, modality gated, and voltage gated

What are the four types of channels?

Neurolemmocyte starts to envelop multiple axons, however because they don't have a myelin there is not sheath

What are the steps of unmyelinated axons?

satellite cells and neurolemmocytes. They both electrically insulate the cell body and regulate the continuous exchange of nutrients and waste products between neuron cell bodies and their environment

What are the two types of glial cells found in the peripheral nervous system (PNS) and what do they have in common?

chemical synapses (most common) and electrical synapses.

What are the two types of synapses in the human body?

Current=Voltage/Resistance. This expression shows that current is directly related to voltage and inversely related to resistance. Thus, a greater current is possible with a larger voltage difference and a lower resistance.

What is Ohm's Law?

1. Nerve signal reaches synaptic knob 2. Voltage gated Ca2+ channels open and Ca2+ enters the synaptic knob and binds with proteins associated with synaptic vesicles 3. Synaptic vesicles fuse with synaptic knob plasma membrane and neurotransmitter is exocytosed 4. Neurotransmitter diffuses across synaptic cleft and attaches to receptors on a muscle

What is the transmissive segment process?

Resting membrane potential (RMP)

When a neuron is a rest. Typically -70 millivolts (mV), but can range between -40 mV and -90 mV. Ions diffuse down their concentration gradient through the membrane that is selectively permeable and allows some ions to pass easier than others. There is an electrical attraction of cations and anions

Chemical synapse

between two neurons and is composed of a presynaptic neuron and post synaptic neurons. May be between the axon of the presynaptic neuron and any portion of the surface of a postsynaptic neuron (dendrite, cell body, or axon), except those regions that are covered by a myelin sheath. Most commonly with a dendrite of the postsynaptic neuron

Electrical synapse

composed of a presynaptic neuron and a postsynaptic neuron physically bound together. Gap junctions are present in the plasma membranes of both neurons and facilitate the flow of ions between the cells. The cells act as though they share a common plasma membrane. Thus, the electrical signal passes between the cells with essentially no synaptic delay.

Role of (Na+/K+) Pumps

contribute approximately -3 mV of the total -70 mV difference by moving more Na+ out of the neuron than K+ is pumped into the neuron. More significant role in maintaining the concentration gradients of both K+ and Na+. These concentration gradients allow for the diffusion of Na+ and K+ as part of the neuron's generation of an electrical current, as described later. (Push 3 positive out and 2 in)

Astrocytes

exhibit a starlike shape due to projections from their surface. These numerous cell processes have contact with both capillaries (smallest blood vessels) and neurons. Most abundant glial cell in the CNS and constitute over 90% of the nervous tissue in some areas of the brain. They nurture, protect, support, and guide neurons

Satellite cells

flattened cells arranged around neuronal cell bodies in a ganglion that physically separate cell bodies from their surrounding interstitial fluid. Regulates nutrient and waste exchange for cell bodies in ganglia

Neurolemmocytes

flattened cells wrap around and insulate axons within the PNS to form a myelin sheath. As with myelin sheaths formed by oligodendrocytes in the CNS, this allows for faster propagation of action potentials. The neurolemmocyte starts to encircle a 1-millimeter portion of an axon. As it continues to wrap around the axon, the cytoplasm and nucleus are squeezed to the periphery of the neurolemmocytes.

Neurofibril Nodes

gaps between the neurolemmocytes

Astrocyte Functions

helps form the blood-brain barrier, regulates interstitial fluid composition, provides structural support and organization to the central nervous system, assists with neuronal development, and replicated to occupy space of dying neurons

Unmyelinated axons

in the PNS and are associated with neurolemmocytes, which help to protect and support the axon. However, no myelin sheath covers them. Thus, the axon merely rests in a depressed portion of the neurolemmocyte, but its plasma membrane does not form repeated layers around the axon. In the CNS, they are not associated with oligodendrocytes

Receptive segment

includes both dendrites and the cell body, which are the regions of the neuron that receive stimuli to excite the neuron. Binding of neurotransmitter released from synaptic neurons; production of graded potentials

Transmissive segment

includes the synaptic knobs and contains voltage-gated Ca2+ channels and Ca2+ pumps. Action potential causes release of neurotransmitter

Initial segment

is conventionally considered to be composed of the axon hillock. This segment contains both voltage-gated Na+ channels and voltage-gated K+ channels. Summation of graded potentials; initiation of action potential

Conductive segment

is equivalent to the length of the axon and its branches called telodendria. Propagation of action potential

Potential energy

is the energy due to position

Kinetic energy

is the energy of motion. Ions can flow across the plasma membrane through open channels, establishing ion currents. It is this movement of ions that is harnessed when electrical signals are transmitted in a neuron.

Presynaptic Neuron

the signal producer

Postsynaptic neuron

the signal receiver or target.

Synaptic Cleft

two neurons are separated by an extremely narrow fluid-filled gap (of about 30 nanometers)

Myelin

is the insulating covering around the axon that consists of repeating concentric layers of plasma membrane of glial cells. Myelination is completed by neurolemmocytes in the PNS and by oligodendrocytes in the CNS. Myelin mainly consists of the plasma membrane of these glial cells and contains a large proportion of lipids and a lesser amount of proteins. Has a high lipid content that gives an axon a distinct, glossy-white appearance and serves to effectively insulate an axon.

Oligodendrocytes

large cells with a bulbous body and slender cytoplasmic extensions or processes. The extensions wrap around and insulate axons within the CNS to form a myelin sheath. This insulation allows for faster propagation of action potentials.

Ependymal cells

line ventricles of brain and central canal of spinal cord and assists in the production and circulation of Cerebrospinal fluid

Choroid plexus

made up of ependymal cells and nearby blood capillaries. They help produce cerebrospinal fluid (CSF), a clear liquid that bathes the external surfaces of the CNS and fills its internal cavities. The cilia of ependymal cells help circulate the CSF.

Voltage

measure of the amount of difference in electrical charge between two areas and represents potential energy.

Pumps

membrane proteins maintain specific concentration gradients by moving substances up (against) a concentration gradient, a process that requires cellular energy.

Current

movement of charged particles across the barrier that separates this charge difference. The greater the movement of charged particles, the greater the _____. Movement of charged particles can be harnessed to do work

Resistance

opposition to the movement of charged particles. This is the barrier between the charged areas.

Myelin sheath

overlapping inner layers of the plasma membrane

Neurilemma

periphery of the neurolemmocyte containing the cytoplasm and nucleus

receptive segment, initial segment, conductive segment, and transmissive segment.

A typical neuron is functionally organized into four segments called?

Activation state

Both the inactivation gate (which remains open) and the activation gate are open (activation gate opens in response to a voltage change); Na+ moves into the cell through the open channel.

role of (K+)

K+ diffusion is the most important factor in setting RMP. It diffuses out of the cell due to its concentration gradient. The diffusion out is limited by the electrical gradient (the pull of the negative RMP on the positive ion. If K+ were the only ion that leaked, RMP would be where the K+ concentration and electrical gradients are at equilibrium (-90 mV)

Role of (Na+)

There are a few Na+ leak channels, Na+ also influences RMP. It diffuses in due to its concentration gradient and the electrical gradient. This small Na+ leakage means RMP is less negative (-70 mV)

Membrane potential

There is an electrical charge difference (an electrical gradient) across the plasma membrane; the cytosol adjacent to the plasma membrane is relatively negative in ¬comparison to the IF outside of the cell.

Voltage-gated channels

These channels are also normally closed, but they open in response to changes in electrical charge (potential) across the plasma membrane. When open, they allow a specific type of ion to diffuse across the membrane.

Leak (passive) channels

These channels are always open, allowing continuous diffusion of a specific type of ion from a region of high concentration to a region of low concentration.

Chemically gated channels

These channels are normally closed. They open in response to binding of a neurotransmitter. When open, they allow a specific type of ion/ions to diffuse across the plasma membrane.

Modality gated channels

are an additional type of channel that is normally closed. These channels open (or close) in response to a stimulus other than a chemical or a voltage change. They are components of sensory neurons, which detect changes in the external or internal environment.

Microglia

typically small cells that have slender branches extending from the main portion of the cell. They represent the smallest percentage of CNS glial cells and are classified as phagocytic cells of the immune system. They wander through the CNS and replicate in response to an infection and protect the CNS against microorganisms and other potentially harmful substances by engulfing infectious agents and removing debris from dead or damaged nervous tissue that results from infections, inflammation, trauma, and brain tumors.